In recent years, optical fibers are developing rapidly in subscriber system communication networks to satisfy to market demand for high speed and high capacity communication. A wavelength division multiplexing (WDM) system is spotlighted as a communication method. In the WDM system, a single mode optical fiber is used for transmitting and receiving on different wavelengths, 1310 nm and 1490 nm. A passive optical network (PON) system is noted in which a central office serves multiple end users. To provide for the PON system, a bi-directional optical module for an optical line terminal (OLT) for transmitting on the wavelength of 1490 nm and receiving on the wavelength of 1310 nm for the central office, and a bi-directional optical module for an optical network unit (ONU) for transmitting on the wavelength of 1310 nm and receiving on the wavelength of 1490 nm for an end user are developing rapidly. Under such circumstances, there is a demand for compact, low-cost ONU modules in vast numbers.
A bi-directional optical subassembly (hereinafter, referred to as “BOSA”) constituting the main part of a bi-directional optical module usually include two packages for a laser diode (LD) and a photodiode (PD) (packaged individually with a can cap called “TOY-CAN”), a fiber pigtail, a WDM filter, and a housing for holding the above components. The can cap for to LD and the can cap for the PD are positioned to form a right angle. Connecting pins for the LD and to PD also form a right angle. The distance between the pins must be long in order to reduce electric crosstalk between the transmitter and the receiver.
The BOSA with two TO-CANs often has a limited degree of freedom in design, and is complicated. In configuration. Further, manufacture of such a BOSA requires a large number of steps, and manufacturing cost cannot be reduced.
To address the above problems, a structure has been proposed in which the LD/PD chips and optical/electric systems therefor a collectively accommodated in a single TO-CAN (for example, see United States Patent Application, Publication No. 2005/0084268).
However, in the structure in which the optical and electric elements for transmitting and receiving are accommodated together in the TO-CAN, optical interference such as optical crosstalk (i.e., stray light) and electric interference such as electric crosstalk are obstacles in providing a compact and low-cost structure.